HTC Desire Internet Usage

Hey.
On the HTC Desire how do I tell if my phone is using the internet? I have 500mb of internet and I dont know if my phone is on the internet after i close the browser. e.g if i just return to the homepage after using facebook, does the browser automatically close and does not count as internet usage anymore?
Thank you.

I Have just gotten my new android phone only to notice there is know easy way of knowing my internet usage .It would be nice if somebody who knows how to program apps for android could make a good one. The issues i have encountered on the existing apps are -

1. no option to set how much internet i have used so far (so if i get the app half way through the month or i need to update it manually i can do so)

2. No way of seeing what each individual app is using when it updates or syncs as the day goes on (because every four hours or whatever its set to my apps check for updates), furthermore, I use the navigator which is like a normal car GPS which uses no internet but because its through google and uses their maps it needs a small amount of internet which i would like to monitor when i leave it running for an hours driving.

3. I recieve unlimited access to certain sites; like my providers website (vodaphone) and others like facebook and twitter which would be nice if they could have a setting to exclude or include them into my usage. (and then if i could manually set my usage, reset it to its proper value if i leave facebooks included usage for a while).

Not sure why I am posting this here, though I thought it would be nice if more people request or are in need of a similar app, for even different purposes to try help me find or promote something. ty.

I just went on the app store, and got myself a copy of 3g watchdog. It confused me at first, then i had a little play with all the settings, simple.
You are right, 3g watchdog works fine. I don't have the pro version though, Which would allow me to keep tolls on individual apps, just the free version.
So i guees one day when I get around to buying apps ill upgrade, It would be much worth it, considering its $3.00 for my 24 month contract; which means i will be on top of my usage for the whole term. Still, I'm lazy and cheap so ill just wait and see which way the cookie crumbles.